Impact of normal ageing and cerebral hypoperfusion on myelinated axons and its relation to the development of Alzheimer’s disease

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Abstract

Cerebral hypoperfusion can occur in normal ageing and is proposed to underlie white
matter disturbances observed in the ageing brain. Moreover, cerebral hypoperfusion
and white matter attenuation are early events in the progression of Alzheimer’s
disease (AD). White matter mostly consists of myelinated axons which have distinct
protein architecture, segregated into defined regions; the axon initial segment (AIS),
the node of Ranvier, paranode, juxtaparanode, and internode. These sites are
essential for action potential initiation and/or propagation and subsequently effective
brain function. At the outset of the studies in the thesis there was evidence that the
different regions within the myelinated axons are vulnerable to injury and disease.
Thus it is hypothesised that in response to normal ageing and/or cerebral
hypoperfusion these structures are altered and associated with cognitive impairment
and that these effects are exacerbated in a transgenic mouse model (APPSw,Ind, J9
line) which develops age-dependent amyloid-β (Αβ) pathology.
The first study aims to investigate the effect of normal ageing and Aβ deposition on
myelinated axons and on learning and memory. To address this, the effects of normal
ageing on the integrity of the AIS, nodes of Ranvier, myelin, axons, synapses and
spatial working memory are examined in young and aged wild-type and
TgAPPSw,Ind mice. A significant reduction in the length of nodes of Ranvier is
demonstrated in aged wild-type and TgAPPSw,Ind mice. In addition, the length of
AIS, is significantly reduced in the aged wild-type animals while the young
TgAPPSw,Ind have significantly shorter AIS than the young wild-type mice. These
effects are not influenced by the presence of Aβ. Myelin integrity is affected by age
but this is more prominent in the wild-type animals whilst axonal integrity is intact.
Moreover, there is an age-related decrease of presynaptic boutons only in the
TgAPPSw,Ind mice. Contrary to the original hypothesis, working memory
performance is not altered with age or influenced by increasing Aβ levels.
The second study aims to examine the effects of cerebral hypoperfusion in
combination with Αβ pathology and/or ageing on cognitive performance and the
structure of myelinated axons. To address this, the effects of surgically induced
cerebral hypoperfusion on the integrity of the nodes of Ranvier, paranodes, myelin,
axons and spatial working memory performance are investigated in young and aged
wild-type and TgAPPSw,Ind mice. A decrease in nodal length is observed in
response to hypoperfusion in young and aged animals. This effect is shown to be
exacerbated in the young TgAPPSw,Ind animals. Moreover, the disruption of the
nodal domain is shown to occur without any gross alterations in myelin and axonal
integrity. It is also demonstrated that in response to hypoperfusion, spatial working
memory performance is defected in young and aged animals of both genotypes. This
deficit is exacerbated in the young TgAPPSw,Ind. The observed changes in the nodal
structure are associated with poor working memory performance indicating
functional implication for the nodal changes.
These data highlight that structures within myelinated axons are vulnerable to ageing
and cerebral hypoperfusion. Therefore, the development of strategies that minimize
injury or drive repair to these regions is necessary together with therapeutic
approaches against the vascular insults that induce hypoperfusion and lead to white
matter attenuation and cognitive decline. In the future, it would be interesting to
investigate how alterations at the AIS/nodes of Ranvier affect neuronal excitability.